Revista Mexicana de Fısica S58 (2) 138–141 DICIEMBRE 2012

Magnetic Properties of CoFe0.5Cr 1.5O4 Nanoparticles

G. Marqueza, V. Sagredoa, C. Marquinab, T.E. Torresb,c, M.R. Ibarrab,c and G.F. GoyacaLab. de Magnetismo, Dpto. de Fısica, Facultad de Ciencias,

Universidad de Los Andes, 5101 Merida, Venezuela.e-mail: gersonmarquez@ula.ve

bInstituto de Ciencia de Materiales de Aragon,Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain.

cInstituto de Nanociencia de Aragon,Universidad de Zaragoza, E-50009 Zaragoza, Spain.

Recibido el 25 de junio de 2010; aceptado el 22 de marzo de 2011

Nanoparticles of CoFe0.5Cr1.504 were synthesized by using the auto-combustion method. Structural and magnetic properties were par-ticularly investigated. The obtained powder was characterized by X-ray diffraction (XRD) and magnetic measurements as a function oftemperature and applied magnetic field. X-ray diffraction data suggested the presence of single crystalline phase corresponding to cubic-spinel structure. The size particle was estimated by the Scherrer equation in 23 nm. The main characteristic of the Field Cooling (FC) andZero Field Cooling (ZFC) curves is the presence of a magnetic transition at about 175 K; below this temperature the compound is magneti-cally ordered, as ferrimagnetic state, and above 175 K the magnetic behavior of the system is suggested that superparamagnetic. At 5 K themixed oxide presents a quite high coercive field, 5.8 kOe.

Keywords: Spinel; nanoparticles; superparamagnetism.

Nanopartıculas de CoFe0.5Cr1.504 fueron sintetizadas mediante el metodo de auto-combustion. Las propiedades estructurales y magneticasfueron especialmente estudiadas. Los polvos obtenidos fueron caracterizados mediante difraccion de rayos-X (XRD) y medidas magneticasen funcion de la temperatura y del campo magnetico aplicado. El perfil de difraccion de rayos-X sugiere la presencia de una sola fase cristalinacorrespondiente a la estructura cubica espinela. El tamano de las partıculas fue estimado mediante la ecuacion de Scherrer, obteniendotamanos de partıcula de 23 nm. La caracterıstica principal de las curvas de magnetizacion, Field Cooling (FC) y Zero Field Cooling (ZFC),es la presencia de una transicion magnetica alrededor de 175 K; por debajo de esta temperatura el compuesto se encuentra magneticamenteordenado, en estado ferrimagnetico, y por encima de 175 K el comportamiento magnetico del sistema se sugiere es superparamagnetico. A5 K el oxido mixto presenta un considerable campo coercitivo de 5,8 kOe.

Descriptores: Espinela; nanopartıculas; superparamagnetismo.

PACS: 61.05.cp; 61.46.Df; 75.50.Gg; 75.50.Tt.

1. Introduction

Nanosized spinel mixed oxides, such as ferrites andchromites, have been extensively studied because of impor-tant physical and chemical properties that make these mate-rials are interesting for their high number of applications ashigh-density information storage, catalysts, ferrofluids, sen-sors, biomedical treatments, etc [1-4]. Very recently it hasbeen found that the cobalt chromite (CoCr2O4) nanostruc-tured presents very interesting magnetic and catalytic prop-erties. One of the possible applications is the elimination ofNOx gases produced in the emission exhaust from diesel ve-hicles [2,5].

The cobalt chromite crystallizes in the well known cubic-spinel structure with general formula AB2O4. The spinelstructure presents two possible arrangements: the first oneknown as normal spinel is characterized byA cations in thetetrahedral sites andB cations in the octahedral positions.A second possible arrangement known as inverse spinel, inwhich theA cations occupies one half of the octahedral co-

ordination sites and theB cations occupies the other half ofthe octahedral sites as well as all of the tetrahedral positions.However, there are many possible intermediate distributionsrepresented by the formula (A1−λBλ)T (AλB2−λ)OO4 whereλ is called inversion parameter [6]. On the other hand, thebulk cobalt chromite presents a magnetic transition around93 K [7-9].

Nanosize spinel oxides have been prepared by vari-ous techniques such as sol–gel, combustion, coprecipitation,hydrothermal process, ball milling and the microemulsionmethod [10-12].

We have considered quite interesting the partial substi-tution of Cr+3 by Fe+3 in the nano-cobalt chromite andcarry out an study on the structural and magnetic changesbecause of the interactions between those three magneticions: Co+2, Fe+3 and Cr+3. For this reason, nanoparti-cles of CoFe0.5Cr1.504 were synthesized by using the auto-combustion method, with nitrates of the cations as the pre-cursors and urea as fuel.

In this work we report on the structure and magnetic prop-erties of CoFe0.5Cr1.504 nanoparticles.

MAGNETIC PROPERTIES OF CoFe0.5Cr1.5O4 NANOPARTICLES 139

FIGURE 1. Schematic representation of synthesis method ofCoFe0.5Cr1.504 nanocompound.

2. Experimental

Nanoparticles of CoFe0.5Cr1.504 were obtained by the auto-combustion method. To get 2.5 grams of final compound,stoichiometric quantities of nitrates: Co(NO3)2·6H2O,Fe(NO3)3·9H2O and Cr(NO3)3·9H2O, and urea CO(NH2)2,were dissolved in desionized water following the procedureschematically illustrated in Fig. 1. The amount of urea used isdetermined by adjusting the stoichiometric coefficientφ=1.5,according to the method proposed by Jainet al. [13].

During constant stirring, ammonium hydroxide(NH4OH) was added dropwise to get a solution with pHabout 7, starting with the precipitation of the final compound.

The obtained powder was characterized by X-ray diffrac-tion (XRD) using a D/Max Rigaku powder diffractometerequipped with a rotating anode and a graphite monochroma-tor to select CuKα radiation to identify the crystalline phasespresent in the samples. Using the data from the pattern ofX-ray diffraction, particle size was estimated by the Scherrerequation [14]:

D =0.9λ

β cos θ(1)

whereD is the particle size,λ is the wavelength of X-rays,β is the half width of intensity maximum peak andθ is thediffraction angle.

The magnetic measurements were made using a SQUIDmagnetometer, MPMS-XL5 of Quantum Design, in the tem-perature range from 5K to 300K and applying a magneticfield up to 50 kOe. In addition, we used a Vibrating SampleMagnetometer EV7 of ADE TECHNOLOGIES to measurehysteresis loops at room temperature by applying a magneticfield up to 20 kOe.

3. Results and discussion

The XDR patterns of the sample of CoFe0.5Cr1.504 nanopar-ticles synthesized is presented in Fig. 2. Comparing theobtained diffractogram with the patterns of the internationalcrystallographic cards of CoCr2O4 and CoFe2O4 (JCPDS:22-1084 y JCPDS: 22-1086) [15,16], it is found that all the

FIGURE 2. Indexed X-ray diffraction pattern of CoFe0.5Cr1.504

nanoparticles.

observed intensity maxima correspond to the characteristicpeaks of CoCr2O4 and CoFe2O4. This means that the sam-ple correspond to a single crystalline phase: the cubic-spinelstructure.

It was found that the unit cell parameter ofCoFe0.5Cr1.504 is 8.347 (1) A, slightly larger than theknown to the CoCr2O4 (8.3299 A) as observed by Maneet al. [17]. This result is reasonable since it is replacingpart of the cations Cr+3 by Fe+3 in the cobalt chromite, andalso ionic radius of Fe3+ (0.645A) is higher than the Cr3+

(0.63 A) [18]. On the other hand, the average particle sizeestimated by the Scherrer’s equation is approximately 23 nm.

In Fig. 3 we show the measures Field Cooling (FC) andZero Field Cooling (ZFC) magnetization of cobalt-chromite.Here we can see that there is a magnetic transition at about175 K, above this temperature the nanocompound is in adisordered magnetic state (superparamagnetic state). In therange of temperatures below 175 K, the compound is mag-netically ordered (ferrimagnetic state).

FIGURE 3. Temperature dependence of the FC and ZFC magneti-zation.

Rev. Mex. Fis. S58 (2) (2012) 138–141

140 G. MARQUEZ, V. SAGREDO, C. MARQUINA, T.E. TORRES, M.R. IBARRA, AND G.F. GOYA

FIGURE 4. Field dependence of magnetization curves forCoFe0.5Cr1.504 nanocompound at different temperatures.

FIGURE 5. Field dependence of magnetization for CoFe0.5Cr1.504

particles at room temperature.

Figure 4 shows the isothermal magnetization curves mea-sures at 5 K, 140 K, 175 K and 220 K, we can observe thatthe M-H curve at 220 K shows a similar behavior to that of asuperparamagnetic system, what makes us infer that at 220 Kthe CoFe0.5Cr1.504 sample is in a superparamagnetic state,which is consistent with the results obtained in the measure-ments of FC and ZFC magnetization. At 5 K we can seea clear hysteresis loop with a coercive field of 5.8 kOe. InFig. 4 can be also noted that in the different M-H curves, themagnetization does not reach saturation with the maximumapplied magnetic field (50 kOe), therefore in Table I insteadof the saturation magnetization we refer to the magnetizationmeasured with the maximum applied magnetic field as themaximum magnetization (Mm).

TABLE I. Magnetic parameters of the CoFe0.5Cr1.504 nanoparti-cles.

T (K) H c(Oe) Mr (emu/g) Mm (emu/g)

5 5800 6.40 13.7∗

140 585 1.06 7.0∗

175 123.3 0.31 5.7∗

220 50.1 0.07 4.1∗

300 64.5 0.03 1.7∗∗

∗Magnetic field up to 50 kOe,∗∗Magnetic field up to 20 kOe

It is possible to notice in Fig. 4 that the coercitivity of thenanoparticles decreases as the temperature increases towardthe blocking temperature. The inserts in Fig. 4 show the partof theM(H) loops in the region of small applied field. Fig-ure 5 shows the hysteresis curve at room temperature, as inthe M-H curve at 220 K is similar to the behavior of a super-paramagnetic system.

Table I shows the magnetic parameters of the nanocom-pound under study, where we can see that the values of coer-cive field (Hc), remanent magnetization (Mr) and maximummagnetization (Mm) at 5 K are greater than those found inmeasurements made at higher temperatures (140 K, 175 K,220 K and 300 K). In addition, it is possible to see that thevalue of these parameters decreases with increasing temper-ature towards the magnetic transition temperature (175 K), aresult which agrees with the findings on measures of FC andZFC magnetization.

4. Conclusions

CoFe0.5Cr1.504 nanoparticles synthesized by auto-combustion method has a single crystalline phase corre-sponding to the cubic-spinel structure. The average particlesize estimated by Scherrer equation was 23 nm. The curvesof magnetization as a function of temperature (FC and ZFC)and depending on the applied magnetic field showed thatCoFe0.5Cr1.504 presents a magnetic transition around 175K.Below this temperature the compound shows a ferrimagneticbehavior and above 175 K it is in the superparamagneticstate.

Acknowledgements

G. Marquez and V. Sagredo are thankful to CDCHT-ULA forproviding financial support for this work through the projects:C-1641-08-05-ED and C-1658-09-05-A.

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MAGNETIC PROPERTIES OF CoFe0.5Cr1.5O4 NANOPARTICLES 141

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